The growing of plants in greenhouses made of plastic or glass is known. A clear cover can be penetrated by both short wave and long wave radiation in the range of visual light. It is also known that greenhouse covers transmit more short wave radiation than long wave radiation, the heating effect of the light in the green houses taking effect in that the short wave radiation is converted to long wave radiation within the greenhouse.
The greenhouse principal has also been used in growing fields in such way that covers have been formed to a bow over the surface of the field or the cover has been applied to the surface of the earth. The cover materials used for such purposes in the fields have been of plastic or paper through which the plants grow. This known technique produces several advantages, namely, temperature increase by using transmitting covers so that short wave radiation is converted to heat, the prevention of evaporation of the water by paper covers, and the hindering of the growing of weeds under covers using a film that transmits light.
The need for weed killers is the same when using clear covers or in the case of growing without any cover, and in some cases, even more so in the case of the use of covers because by using a clear cover the humidity is condensed on the inside of the film and good conditions are formed for the growth of weeds around the growing plants. Furthermore, the fact that the earth is exposed between the films as a result of earth terracing used to fasten the films, results in an improvement of the growing conditions for weeds.
Previously known films have been decomposed to small pieces as a result of ultraviolet radiation. Attempts have been made to achieve films that are degraded by ultraviolet radiation to such extent that microorganisms may be able to continue the degradation and to completely degrade the films. However, degradation in these previously known films has not been complete. In practice, the edge of the cover film covered by the earth remains in the field because the UV-radiation cannot transmit through the earth to degrade the film. Even a little piece of earth or a dust layer on the film hinders the energy of the UV-light to enter the film, which is needed for the degradation of the polymer bonds.
In theory, the films are degraded by means of UV-light even after the plants begin to cover the same, and further in theory, the degradation being to such small pieces that they do not cause problems in the field. In practice, however, the degradation is not complete as a result of UV-light, which results in more and more plastic material in the field because the synthetic polymer itself in the so called composite films is not biologically degraded. The synthetic polymers do not absorb water and since the biological degradation takes place by means of enzymes produced by microbes, no biological degradation can occur because these enzymes act only in the presence of water.
Synthetic polymers, the molecules of which consist of, for example, about 12,000 successive carbon atoms, should be degraded to about 500 pieces with the lengths of about 10 carbon atoms before the rate of biological degradation has any importance whatsoever. However, the structure of the plastic films is already essentially changed in the stage when the polymer has been degraded to pieces containing thousands of carbon atoms. A plastic film, degraded in this way, could cause serious environmental problems. As a result, the use of films of this type has recently decreased. Problems have also arisen from the toxic residues that are caused by such degradation.
Methods are also previously known in which the film is removed from the field after the growing season has ended. Use of such films is, however, very expensive. Furthermore, in previously known methods, it is mainly thin films that have been used, because their preparation is much cheaper. However, it is difficult to remove such films from the field because the films are easily decomposed. In previously known solutions, the film has covered only about 50-70% of the growing surface because it has become necessary to leave exposed earth between the films for the fastening terracing.
In the our earlier Finnish Application FI-891905, the problem of degradation of the film has been improved by fastening the film to the growing surface by quiltings, as a result of which the entire growing surface is covered by the film. In the solution of this Finnish application, the degradation of the film by means of UV-light is improved because the entire film is on the ground. However, the problem remains that even by use of a UV-degradable film, more and more plastic is retained in the field, because these films are not biologically degraded.
Biologically degradable materials can be degraded biologically as a result of their chemical structure, by the act of microorganisms, such as mould, fungi and bacteria, when they are put in contact with the earth, or by being brought into contact with microorganisms in another manner, under conditions in which the microbes can grow. The term "biologically degradable" is used herein to refer to degradation of the type in which the degradation takes place by the act of living organisms such as microorganisms. The term "degradable" per se, is used with reference to the degradation, of for example, ethylene polymers which, by the act of different additives or other substances degrade into smaller pieces, without microorganisms effecting such degradation.
Attempts have been made to study the biological degradation of plastic films, and even to provide this in different ways, for example, by means of mould cultures. (Compare with ASTM-Standard, STM G 21-70 1980, that has been used in the investigations of so called biological degradation of plastic materials). The growth of mould on plastic films has, however, not shown anything about the biological degradation despite the growing of the mould. The growth of mould on plastic films has been considered to be in correlation to the amounts of additives in the films, with no influence on the synthetic polymer itself.
Generally it can be stated that the film material is degraded into pieces if it does not contain antioxidants, but contains, for example UV-catalysts which break the C--C bond of the synthetic polymer molecule. If the plastic molecules contain double bonds, these are degraded with less energy, even without any catalyst.
The biological degradation of the synthetic material requires hydrophilic water soluble groups. The polymer must be broken so that such hydrophilic chemical group is formed that can be enzymatically degraded, for example a carbonyl or carboxyl group. The degradation product of the biological degradation of the film must be water, carbon dioxide and biomass.
Several attempts have been made to prepare such biologically degradable films that consist of a combination of a synthetic polymer and a biopolymer in which a catalyst sensitive to UV-light has generally been added. A substance that degrades synthetic polymers by using light as catalyst is known, for example, from the patent publication EP-230143.
It has been thought that synthetic plastic material is able to absorb water if hydrophilic groups are included therein by means of a biologically degradable polymer. Starch is the cheapest biologically degradable polymer for this purpose, the use of which, in addition, should decrease production costs of the film because its price is lower than that of the polyethylene. Since the gelatinized starch alone forms a very brittle film, that is sensitive to water, it is commonly known that the starch must be combined with other substances that can be used in the film in order to arrive at a satisfactory product.
Polyethylene (PE) is the most commonly used synthetic polymer for preparing films with desired physical properties. Early attempts to produce PE-films by blowing technique from compositions with a high proportions of starch, &gt;30% (w/w) have, however, not been successful. The reason is that starch is a very coarse material (particle size 20-150 .mu.m) that interferes with the making of thin films. Furthermore, in the blowing technique, the starch particle and the molten plastic mass move with different speeds in a blowing technique carried out at normal blowing temperatures (170.degree. C.) which results in brittle and breakable material with holes being formed. In other words, it has not been possible to make such film by blowing because the film necessarily becomes too thick.
Attempts have also been made to bring chemical bonds about the starch particles in order to facilitate the mixing of the synthetic polymer therewith. Such films have been described, for example in the following patent publications, U.S. Pat. No. 4,337,181, GB-1,487,050 and GB-1,485,833. In these known films, the enzymes can in theory degrade the material because the material is, at least in theory, to some extent wet. However, the technique is very expensive and furthermore, the material has several disadvantages, namely, poor tensile strength, thick films cannot be produced and the films do not stretch.
Attempts have also been made to add other reactive groups into the film material, for example double bonds. When the material contains double bonds and it reacts with oxygen and a metal catalyst, (for example, Fe3+), reactive peroxides (--C--O--O --C--) are formed. Thus, free oxygen atoms and radicals are formed resulting in the bonds between the carbon atoms being degraded, and for example carboxyl groups and smaller carbon-hydrogen chains are formed. The phenomenon has been used in films that contain a metal catalyst (note for example, Patent Publication EP-86 310 154.9). When the resulting film material contains carboxyl groups (RCOOH), the same can be degraded by the action of enzymes from the microorganisms if surrounded by water. In other words, reactive groups and catalysts have been added to the film material by means of which biologically degradable RCOOH-- groups are obtained under given conditions. However, these materials are very expensive to produce.
In U.S. Pat. No. 4,337,181, starch, ethylene-acrylic acid copolymers, and optionally polyethylene have been mixed and blowed to a film by using additives that neutralize a portion of the functional acid groups of the copolymer. This method makes the use of wet starch possible, but requires expensive additives. According to the EP Patent Application 0 230 143, the photodegradation must be facilitated by using photodegradable substances that comprise a photosensitive substance and an ethylene/carbon monoxide copolymer. Preferably, the photosensitive substance is a heavy metal dithiocarbamate or a heavy metal dithiophosphonate. As previously stated, ethylene copolymers that contain carbonyl groups are degradable by means of ultraviolet light, but the life span of these copolymers, is not sufficiently long.
Additionally, U.S. Pat. No. 3,901,838, mentions films that consist of a biologically degradable thermoplastic polymer and a degradable ethylene polymer, the mixing being carried out in a conventional mixture and the powdering in a mill. British Patent No. 1,483,838, teaches a biologically degradable film comprising a biologically degradable substance that is homogenously dispersed in a material forming a non-biologically degradable film that is not dissolved in water, the biologically degradable substance being present in an amount of 40-60% of the weight of the film material. In this solution, the biologically degradable substance is a finely divided substance that absorbs water, the film being made of an aqueous dispersion of these substances. The film is, in other words, made from a dispersion in organic solvents or in aqueous systems and because of the physical properties, the same cannot be used as a growing film.
In summary, it can be stated that the degradation of cover films is a two-pronged problem. On the one hand, it is desired that the film not be degraded as long as it is used. On the other hand, when the use of the film is complete, it is desired that the same be returned to the ecosystem in a form that does not cause harm to the environment. Under these conditions, the macromolecules should be split into smaller compounds that in turn can be used as food for the organisms through which they should be returned to the food cycle.
For the most part of commercial vinyl plastics, polyethylenes, polypropylenes, polystyrenes, polyvinyl chlorides and aromatic polyesters withstand microbial degradation. The only polymers that are biologically degraded are highly oxidated products such as cellulose, aliphatic polyesters and polyurethanes based on polyester. Since these can be degraded to water soluble short chains, they can be used as food by microbes. Treatments that lower the molecular weight and perhaps also change the chemical structure, expose the polymers to the degradation action of the microbes. When, for examples polyethylene is acidified with nitrous acid, waxy compounds on which thermophilic mushrooms can grow, are obtained. An intensive ultraviolet radiation can also cause chemical changes in the plastics as, for example, forming of carbonyl groups in which ketones are a part of the metabolism of the microorganism.
UV-radiation, photo degradable additives, morphological surface, additives, antioxidants and molecular weight all have an influence on the biological degradation of polyethylene. The biological degradation of paraffin can be compared with the degradation of polyethylene. In the beginning of the degradation, the main influencing factor is the UV-light and/or oxidation agents. However, once the carbonyl groups have been produced, the microorganisms attack the same and degrade the polyethylene chain to shorter fractions, carbon dioxide and water being the end products. The biological degradation and the ambient factors have a strong synergism, which is why the result can never be explained by only one factor. This is because the degradation is a combined result of factors including temperature, UV-light, water, the microbes and their foods. The presence of water is always a condition for biological degradation.